The present invention relates to an ultrasonic liquid ejecting unit for discharging atomized liquid droplets and a method for making the unit. The invention is useful for universal applications including fuel burners and printers.
A piezoelectric oscillating system for effecting atomization of liquids is described in U.S. Pat. No. 3,738,574. Such a piezoelectric oscillating system comprises a piezoelectric transducer mechanically coupled by a frustum to a vibrator plate for inducing bending vibrations therein, a fluid tank and a pump for delivering fluid to the vibrating plate which is disposed at an oblique angle with respect to the force of gravity above the tank. A wick is provided to aid in diverting excess liquid from the plate to the tank. The frustum serves as a means for amplifying the energy generated by the transducer. To ensure oscillation stability, however, the frustrum needs to be machined to a high degree of precision and maintained in a correct position with respect to a conduit through which the pumped fluid is dropped on the vibrator plate and the amount of fluid to be delivered from the pump must be accurately controlled. Further disadvantages are that the system is bulky and expensive and requires high power for atomizing a given amount of liquid. In some instances 10 watts of power is required for atomizing liquid of 20 cubic centimeters per minute, and yet the droplet size is not uniform.
U.S. Pat. No. 3,683,212 discloses a pulsed liquid ejection system comprising a conduit which is connected at one end to a liquid containing reservoir and terminates at the other end in a small orifice. A tubular transducer surrounds the conduit for generating stress therein to expel a small quantity of liquid through the orifice at high speeds in the form of a stream to a writing surface.
U.S. Pat. No. 3,747,120 discloses a liquid ejection apparatus having an inner and an outer liquid chamber separated by a dividing plate having a connecting channel therein. A piezoelectric transducer is provided rearward of the apparatus to couple to the liquid in the inner chamber to generate rapid pressure rises therein to expel a small quantity of liquid in the outer chamber through a nozzle which is coaxial to the connecting channel.
While the liquid ejection systems disclosed in U.S. Pat. Nos. 3,683,212 and 3,747,120 are excellent for printing purposes due to their compact design, small droplet size and stability in the direction of discharged droplets, these systems have an inherent structural drawback in that for the liquid to be expelled through the nozzle the pressure rise generated at the rear of liquid chamber must be transmitted all the way through the bulk of liquid to the front of the chamber. As a result, if the liquid contains a large quantity of dissolved air, cavitation tends to occur producing bubbles in the liquid.
Copending U.S. patent application Ser. No. 434,533 filed Oct. 14, 1982 by N. Maehara et al, titled "Arrangement for Ejecting Liquid, and assigned to the same assignee of the present invention discloses a liquid ejecting device comprising a housing defining a liquid chamber, a ring-shaped piezoelectric transducer and a vibrating member secured to the transducer in pressure transmitting relationship with the liquid in the chamber. Further copending U.S. patent application Ser. No. 458,881, filed Jan. 17, 1983 by N. Maehara, titled "Ultrasonic Liquid Ejecting Apparatus" and assigned to the same assignee as the present invention also discloses a a similar liquid ejecting device in which the vibrating member is excited at a resonant frequency thereof. These copending U.S. applications eliminate the problems associated with the aforesaid U.S. patents. However, problems still exists in these copending applications in that the vibrating member is cemented by a solder to adjacent surfaces of the transducer and the housing and the solder tends to flow outside the periphery of the contact surfaces. This creates an imbalance in the vibration system, causing nonuniform oscillation wave patterns. Furthermore, the adjacent surfaces of the components fail to provide affinity to soldering material, so that they are not satisfactorily wetted by the molten solder and voids occur between them.